With respect to the method for producing a porous coal-based solid fuel, for example, a method described in Patent Literature 1 is conventionally known. This method will be roughly described in reference to FIG. 8. Porous coal (raw material coal) is pulverized in a pulverization step, and then mixed with a mixed oil containing a heavy oil and a solvent oil in a mixing step to prepare a raw material slurry. The raw material slurry is preheated, and then heated in an evaporation step to promote dehydration of the porous coal while impregnating the mixed oil into pores of the porous coal to obtain a dehydrated slurry. Thereafter, a resulting upgraded porous coal and the mixed oil are separated from the dehydrated slurry in a solid-liquid separation step, and the separated upgraded porous coal is dried in a final drying step. The dried upgraded porous coal is subjected to cooling and Briquetting as requested to obtain a solid fuel. On the other hand, the mixed oil which is recovered in the solid-liquid separation step and the final drying step is circulated and carried to the mixing step for preparing the raw material slurry, and reused therein as circulating oil.
In the above-mentioned method, since the upgraded porous coal separated in the solid-liquid separation step is generally dried with a carrier gas in the final drying step, vaporized mixed oil and dust coal of the porous coal are consequently included in the carrier gas after drying. Therefore, prior to the reuse of the carrier gas, the vaporized mixed oil in the carrier gas is condensed and removed by cooling, and the dust coal is captured and removed by atomization of the mixed oil. As a result, the dust coal is included in the recovered mixed oil in relatively large quantity.
In a drying device using carrier gas (CG), for example, as shown in FIG. 9, an upgraded porous coal cake 52 separated in the solid-liquid separation step is heated to vaporize the mixed oil, particularly, the solvent oil in the cake in a dryer 51. Simultaneously, the vaporized oil is transferred and removed from the dryer 51 over the carrier gas (CG) to thereby obtain an upgraded porous coal 53. Since the carrier gas (CG) including the vaporized oil entrains also the dust coal, the dust coal is removed in a dust collecting device 54 as requested. However, generally, the dust coal cannot be sufficiently removed even by the dust collecting device 54. Therefore, in a gas cooler 55, the dust coal in the carrier gas is captured and removed by atomizing the mixed oil as circulating oil (CO) while condensing the vaporized oil by cooling. Consequently, the dust coal is included in a recovered mixed oil 56 in relatively large quantity.    Patent Literature 1: Japanese Patent Application Laid-Open No. Hei7-233383